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Why Does Our Metabolism Slow Down With Age? · So That's Why

• 0:04
  Jen: Welcome to the So That's Why podcast, where we unpack the
• 0:07
  science behind everyday questions.
• 0:09
  I'm Jen.
• 0:10
  Matt: I'm Chris.
• 0:11
  And I'm Matt.
• 0:12
  Jen: Okay.
• 0:13
  I'm sure loads of listeners can relate to today's topic, especially if you
• 0:18
  have ever uttered the words, "My metabolism has definitely slowed down."
• 0:23
  So why does our metabolism slow with age?
• 0:27
  Matt: Well, I am definitely one of the people that says, "My
• 0:30
  metabolism has slowed down." It's one of my favorite things to say.
• 0:34
  It's a great excuse I've found as I'm getting older.
• 0:36
  Jen: Definitely.
• 0:38
  Chris: Well, there's a rather large piece of research with
• 0:41
  an awkward answer for you, I'm
• 0:42
  Matt: afraid.
• 0:43
  I'm not interested.
• 0:43
  Let's end the podcast right now.
• 0:45
  Jen: And normally we'd make everyone wait, but this one works
• 0:49
  better the other way around.
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  So Chris, hit us.
• 0:52
  Chris: Yeah,
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  Jen: so
• 0:54
  Chris: for the fact fans, um, for most of our adult life, metabolism
• 0:58
  doesn't actually sh- slow down at all, despite what we all naturally think.
• 1:02
  So between the age of 20 and 60, metabolic rate is essentially flat.
• 1:08
  Matt: Flat, as in nothing happens.
• 1:12
  I mean, to be fair, like I said, I've been blaming my metabolic rate for
• 1:15
  a fair, fair number of years now.
• 1:16
  Jen: Yeah, and you've been blaming an innocent party, so it's like accusing
• 1:20
  a housemate who moved out years ago of finishing all the biscuits.
• 1:23
  Matt: They did finish- … the biscuits.
• 1:26
  I'm convinced of it.
• 1:27
  Anyway, so, uh, I guess then, am I the suspect?
• 1:32
  Is it me, half nine at night with the fridge door open, you know, with the
• 1:36
  excuse, "Well, it's been a bit of a day"?
• 1:38
  Chris: Uh, before I continue, just for the record, I'd like it
• 1:41
  noted, I had nothing to do with the disappearance of your biscuits.
• 1:44
  Did I, Matt?
• 1:46
  Jen: Noted.
• 1:47
  Okay.
• 1:48
  So let's get into this properly because the story is far better than the myth.
• 1:53
  Chris: So in this study, the test is called, um, doubly labeled water, and
• 1:58
  it's the gold standard for measuring total energy expenditure in real life.
• 2:02
  And the reason it's called doubly labeled water is because the water
• 2:05
  itself contains heavy forms of hydrogen and oxygen, which are then tracked or
• 2:11
  labeled, and the rate those isotopes leave your body tells the researchers
• 2:16
  how much CO2 you're producing, and therefore how much energy you're burning
• 2:21
  Jen: And crucially, it isn't a treadmill test.
• 2:24
  It's real life over days, including the bits when you're asleep or
• 2:29
  arguing about who's turned the thermostat up or down in your house.
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  Do you ever have that in your house?
• 2:33
  Yeah,
• 2:33
  Matt: no.
• 2:33
  Jen: You
• 2:36
  Chris: know, we've got one person on the graph for turn it up
• 2:38
  Jen: and one of us- Yeah,
• 2:39
  Chris: yeah
• 2:39
  … Jen: for
• 2:39
  Matt: turn it down.
• 2:40
  I just have a mobile app, I win every time.
• 2:42
  Chris: Yeah, and on this study, they, um, they actually did it on over 6,000
• 2:46
  people, and they're aged from eight days old all the way to 95 years old, um,
• 2:52
  across, I think it was 29 countries.
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  Let's be honest, that's quite a decent cross-section of the population, isn't it?
• 2:57
  Jen: Yeah, it's quite robust, that study, isn't it?
• 2:59
  Matt: It is.
• 2:59
  I wanna know who had the conversation with the parent of
• 3:02
  an eight ye- eight-day-old baby.
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  "Do you mind?"
• 3:04
  Jen: I know.
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  And they were actually in the right head space to make a decision to say yeah.
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  Chris: I know, and as it happens, I mean, the, the baby turns out to be the
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  most impressive person in this study.
• 3:13
  And I don't mean because they were kind of mature enough to actually
• 3:16
  sign off their own consent.
• 3:18
  Um, energy expenditure peaks at around eight or nine months old, when babies
• 3:23
  are burning roughly 50% faster than an adult, kind of relative to body size.
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  Matt: 50%.
• 3:29
  They can't even hold their own head up properly, and they
• 3:33
  seem to be out metabolizing me.
• 3:35
  Jen: Yeah.
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  Well, it's the building, which is, that's the most expensive
• 3:39
  thing that the body ever does.
• 3:40
  But you know, after that, it declines through childhood and adolescence, and
• 3:46
  then settles around, like, the age of 20.
• 3:50
  Chris: I know.
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  And then from 20 to 60, basically nothing.
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  No dip in the 30s, no crash in the 40s.
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  There's no measurable effect of menopause or pregnancy either once
• 4:01
  you account for size and composition.
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  So-
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  Jen: People will not wanna hear this, will they?
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  Matt: Yeah, no, I know.
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  No.
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  And what does accounting for size and composition actually mean?
• 4:12
  Chris: That sounds like you're questioning the study there, but-
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  Matt: I, I-
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  Jen: This was a robust
• 4:15
  Chris: study.
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  Yeah.
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  Matt's trying to find holes.
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  Um, in the study, they corrected for how much of you there is and how much
• 4:23
  of that is muscle rather than fat.
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  And basically, a bigger body burns more in total.
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  So without that correction, you're just measuring size.
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  Jen: And once you correct for it, so the rate per kilo of tissue, it
• 4:36
  remains steady across the four decades.
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  So your machinery isn't getting worse with age.
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  There may simply be less of the expensive machinery in your body.
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  Matt: And those are two completely different problems though, aren't they?
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  One is your body failing, the other maybe is your body composition changing.
• 4:54
  Jen: Mm.
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  Chris: Yep, and of course, only one of those is a fair description of
• 4:57
  what actually happens in midlife.
• 4:59
  Jen: Which brings us to the awkward question, if the rate
• 5:03
  is flat, why do so many people gain weight in that exact window?
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  Matt: And I'd like to state for the record that I know the
• 5:12
  answer, I just don't care for it.
• 5:16
  Chris: Well, in that case, I'll say it gently and- … diplomatically.
• 5:19
  It's about, let's call it energy balance.
• 5:22
  So you get more energy going in than going out, and of course, this is
• 5:26
  sustained over years rather than weeks.
• 5:29
  Jen: So there's a professor of nutrition at King's College London called Tom
• 5:34
  Sanders, and he said, "Findings like these support the view that rise in
• 5:38
  obesity is driven by excess energy intake rather than falling energy expenditure."
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  Matt: And the thing there is that nobody makes that decision once.
• 5:50
  Jen: Mm.
• 5:51
  Matt: It's a few hundred small decisions that I guess we're
• 5:54
  just not keeping score of.
• 5:56
  Jen: No.
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  And I'd rather people heard that as good news, because everything
• 6:01
  that is the problem is something you can actually influence.
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  Your muscle mass, how much you move, and sadly, what's on your plate.
• 6:10
  Yeah.
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  Matt: Which is- Or what's in your plate … what you said.
• 6:13
  Mm-hmm, yeah.
• 6:14
  Well, yeah, I, I guess I could look at it as like it's oddly freeing, right?
• 6:18
  Mm. I generally file this under the things that happen to you, you know, a bit like
• 6:22
  gray hair and your knees sort of making that noise when you climb the stairs.
• 6:26
  Chris: I thought that was just me.
• 6:29
  That, that, that grating noise.
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  Where is that noise coming from?
• 6:30
  Yeah.
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  Uh, but after 60, something actually does change, and that same
• 6:35
  study is very clear about this.
• 6:38
  Matt: Yeah, so there's a real decline, and it just happens about 30 years
• 6:42
  later than we actually think it does.
• 6:44
  Well, certainly the internet does anyway.
• 6:45
  Jen: Yeah, and that decline is only about 0.7% a year.
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  It's tiny, but it adds up.
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  So by the age of 90, your total energy expenditure is around
• 6:58
  26% lower than in mid-life.
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  Matt: And 26% is not nothing.
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  Chris: No, it's substantial.
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  And, um, the mechanisms are where this gets good, and there are four
• 7:09
  things all happening at once here.
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  Jen: Okay, so start with the big one, because it's also the one
• 7:16
  people have most control over.
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  Chris: Yeah, so it's what's known as sarcopenia, and this is, um, age-related
• 7:23
  loss of skeletal muscle mass and strength.
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  So inactive adults can lose 3 to 5% of their muscle per decade from their 30s,
• 7:32
  and that accelerates through your 40s.
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  Matt: Yeah, so the muscle starts going long before the
• 7:37
  metabolic rate drops or goes.
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  Jen: Yeah, and that matters because muscle is metabolically active tissue,
• 7:43
  so it costs energy to maintain, even when you're sitting completely still
• 7:49
  Chris: Yeah, absolutely.
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  And, um, there was a, a systemic review of resting metabolic rate in older adults,
• 7:56
  and it, uh, it found the drop was m- predicted far more strongly by muscle
• 8:01
  mass loss than by total body weight or anything else about body composition.
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  So in all those studies, muscle was the main player.
• 8:09
  Matt: So keeping muscle isn't really just a gym thing then, it's also about how
• 8:14
  much energy your body burns while resting.
• 8:17
  Jen: Yeah, so think of it like, um, the standing charge on your energy bill.
• 8:21
  Muscle is the bit you pay for whether or not you switch any energy on in the house.
• 8:26
  Matt: The standing charge.
• 8:27
  I like that analogy.
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  So lifting weights then can make a huge difference as you get older.
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  Jen: Mm.
• 8:32
  Chris: Yeah, and it's not always the, as I say, it's not always just about
• 8:34
  the gym, it can be little things.
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  So lifting those bags of groceries on the weekly shop, it counts as something.
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  It's not much, but it's at least something.
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  Better than nothing.
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  Matt: You keep telling yourself
• 8:44
  Chris: that.
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  You don't see
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  Jen: the bags I've got to carry home.
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  Well, it depends what's in your shopping bags really.
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  Okay, so the second mechanism, and this is one you can't lift your way out
• 8:54
  of, is the mitochondria in your body.
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  Matt: The powerhouse of the cell.
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  Chris: You were listening at, uh, secondary school- Yeah … biology.
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  Uh, but that's the one, yeah.
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  And, um, it's, it's a fair description as far as it goes,
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  the kind of, what did you say?
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  The powerhouse- The
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  Matt: powerhouse of the cell.
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  Chris: I won't say it in that typical tone.
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  I was gonna say,
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  Jen: you have to say it in that voice.
• 9:16
  Chris: Um, where were we?
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  Yeah, the mitochondria.
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  So the my- the mitochondria generates ATP, or adenosine triphosphate, which is the
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  actual energy currency your cells spend.
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  Jen: But over a lifetime, the mitochondrial DNA, they get mu- it can get
• 9:31
  mutations and oxidative damage, so they start to make the ATP less efficiently.
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  And sadly, that is one of the core hallmarks of aging.
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  Chris: It is, and just to add to that, of course, the quality control process
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  that clears damaged mitochondria, which is called mi- mitophagy, uh, that
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  itself becomes less effective with age.
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  So the faulty ones accumulate and the problem compounds.
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  Matt: So it's wear and tear, plus the, I guess the maintenance
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  team that's also slowing down.
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  Jen: Yeah, and it hits the hardest in the tissues with the highest
• 10:03
  energy demand, like your muscles, your heart, liver, your brain.
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  Chris: That's right.
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  And then we, we're onto the third, which is, uh, hormones.
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  So the growth hormone, uh, IGF-1, and testosterone, they all decline
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  alongside changes in thyroid function and insulin sensitivity.
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  So these are all signals that tell your body to build tissue.
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  Jen: So the building materials still arrive, the instruction to
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  use them is just a little bit quiet.
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  Matt: Dun, dun.
• 10:32
  Okay, this is great news, this.
• 10:34
  What's the fourth one?
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  Chris: Well, I wanna test you on one to three f- first to see if you
• 10:38
  Matt: have retention.
• 10:38
  Okay, so muscles-
• 10:39
  Jen: Yeah
• 10:41
  Matt: ATP- M- … mitochondria Mitochondria,
• 10:42
  Jen: yeah
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  … Matt: uh, hormones.
• 10:43
  Jen: Well done.
• 10:45
  Chris: Wow.
• 10:45
  I, I, yeah.
• 10:48
  There you go.
• 10:49
  Matt: Hey, never studied, whatever.
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  In
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  Chris: that case, we'll go on to the fourth.
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  Um, and this is actually the one that I, I find the most striking.
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  So after 60, that energy expenditure falls by more than you can explain
• 11:00
  by just muscle loss and fat alone.
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  Jen: Which means the tissues that, that's left is doing less per kilo.
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  So it isn't just that there's less of you working, the cells
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  themselves are running slower.
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  Matt: So using your machinery, there's less machinery.
• 11:16
  Uh, but after 60, I guess the machinery changes too?
• 11:21
  Jen: Then there's one more piece, 'cause there always is.
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  Um, it's why- Inevitably … the practical advice looks the way it
• 11:27
  does, and that is anabolic resistance.
• 11:30
  Matt: Which sounds like something a fitness app would charge
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  you a monthly amount for.
• 11:34
  Jen: Yeah.
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  Chris: Yeah, but anabolic resistance also sounds like the, the force
• 11:37
  that we need to fight the AI robots.
• 11:40
  Release the anabolic
• 11:42
  Matt: resistance.
• 11:44
  You can tell me what's calling right now, the anabolic resistance movie.
• 11:48
  Yeah.
• 11:48
  Chris: Absolutely.
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  And, um, anabolic resistance, it- its role is kind of, it's simpler
• 11:52
  than the name might suggest.
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  So, um, aging muscle responds less strongly to the two things
• 11:58
  that normally trigger it to build, eating protein and using the muscle
• 12:02
  Jen: And given, a given amount of dietary protein produces a smaller
• 12:07
  muscle-building response in an older adult than a younger one, so you need
• 12:12
  a bigger nudge for the same effect.
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  Matt: I've been married 28 years.
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  My wife will agree, you need a bigger nudge for the same effect.
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  So it hasn't just stopped working.
• 12:24
  I guess, in my language, the volume just needs turning up.
• 12:27
  Jen: Mm.
• 12:27
  Chris: That's right, and, um, when that doesn't happen, you can end up with
• 12:31
  what research called sarcopenic obesity, where the muscle loss and fat gain happen
• 12:36
  together, hence why we need this, like-
• 12:37
  Matt: Mm
• 12:38
  … Chris: anabolic resistance.
• 12:41
  Matt: Dun, dun, dun.
• 12:41
  We really do.
• 12:43
  Jen: And so when you, you know, gain fat and muscle together, this carries
• 12:48
  a higher rate of metabolic disorder and a higher mortality risk than either
• 12:53
  alone, and it's self-reinforcing because less muscle means less strength, less
• 12:58
  activity, which costs you more muscle.
• 13:01
  Matt: Yeah, and that's a loop that only runs one way, unfortunately, unless,
• 13:04
  I guess you interrupt it somehow.
• 13:07
  Jen: Okay, so let's talk about what does interrupt it, starting with the strongest,
• 13:11
  which isn't a supplement or a diet.
• 13:14
  Chris: That's it, and we've mentioned it a few times now.
• 13:16
  It's about resistance, and specifically resistance training.
• 13:20
  So this is the, the kind of single most evidence-backed intervention because
• 13:24
  it targets the biggest modifiable driver here, which is muscle loss.
• 13:29
  Matt: So is there then an age where it stops being worth starting?
• 13:33
  So if you're listening to the show and you're like, "I don't
• 13:35
  do weights or any resistance training, am I too old to start?"
• 13:40
  Jen: No, and that's actually the most encouraging part.
• 13:43
  So your muscle protein synthesis machi- machi- meh- … machinery
• 13:49
  It stays responsive to a training stimulus up to, like, very old age.
• 13:55
  Chris: That's right, and studies show it improves the efficiency of everyday
• 13:58
  movement in older adults, and preserves or builds lean mass well into later life.
• 14:03
  So, I mean, we joked about kind of picking up heavy bags
• 14:06
  of groceries, but this counts.
• 14:08
  Jen: There's truth in it.
• 14:08
  Chris: So I think the, the bottom line is don't stop.
• 14:11
  If you can pick things up, don't stop- Yeah
• 14:13
  … Matt: regardless of your age.
• 14:13
  Yeah, yeah.
• 14:13
  It all helps.
• 14:14
  Start if you haven't done so already.
• 14:15
  Jen: Yeah.
• 14:17
  Okay, so there's weights, and then there's protein to consider, and this
• 14:20
  is where the numbers have changed.
• 14:22
  So the recommended daily allowance of, like, 0.8 grams per kilo of
• 14:27
  body weight a day is now widely considered too low for older adults.
• 14:33
  Chris: It is, and these recommendations are always changing, aren't they?
• 14:35
  But at time of recording, it's currently sitting at- Yeah … about 1 to 1.2 grams
• 14:42
  per kilogram of body weight as standard, and 1.2 to 1.5 grams per kilogram of
• 14:48
  weight if you're training as well.
• 14:50
  Yeah.
• 14:51
  Maybe more if you're training heavily, like, you know, like me.
• 14:53
  Matt: Like, obviously, like, yeah.
• 14:54
  I'm just, I- I thought you were on two to two and a half grams of protein.
• 14:56
  Yeah.
• 14:57
  Chris: Yeah.
• 14:58
  Matt: Uh, but, yeah, I guess doing some quick maths, if you're around 70 kilos,
• 15:02
  that's about 80 or 90 grams a day.
• 15:04
  Jen: Yeah, roughly, and the per meal matter, number matters just as much
• 15:10
  because maximal protein synthesis in older adults takes around 25 to 40 grams
• 15:16
  of a good quality protein in a sitting, against about 20 in younger adults.
• 15:21
  Chris: Mm. Yeah, and something else worth keeping in mind is, um, the option
• 15:25
  of spreading it evenly across three meals, which stimulates more muscle
• 15:28
  protein synthesis over a kind of 24-hour period than loading it all at once.
• 15:33
  Mm. Which does go against the, the way a lot of people do, do it, doesn't it?
• 15:35
  Jen: Yeah.
• 15:36
  Matt: Yeah, it does, because I guess most of us, I don't know, we toast something
• 15:40
  beige at lunch, and then have the enormous protein-shaped dinner of an evening.
• 15:44
  Jen: I'm feeling quite self-righteous at the moment, 'cause I do
• 15:47
  have a good protein breakfast.
• 15:48
  Matt: Okay, Jen, you do know, right, there's no protein in a cup of tea?
• 15:52
  I just wanna point
• 15:53
  Jen: that out.
• 15:54
  Chris: I can see the product development team behind Matt saying- … protein
• 15:59
  tea, yep, get it on the list.
• 16:01
  I don't wanna be the one that does the lab trials on the protein tea
• 16:03
  Matt: Yeah, no, that
• 16:04
  Chris: would be nasty
• 16:04
  this is not gonna be very pleasant, is it?
• 16:05
  Yeah,
• 16:05
  Jen: yeah.
• 16:05
  No.
• 16:06
  Chris: That's a big teabag as well.
• 16:07
  Yeah.
• 16:10
  Um, Captain Sensible moment, I'm afraid.
• 16:13
  Dun, dun, dun.
• 16:13
  I think it's the first one of this episode, so I haven't been too bad.
• 16:15
  Jen: Yeah, people have been waiting for it, I'm sure.
• 16:17
  Chris: I know, yeah.
• 16:18
  I need a, I want, like, a lanyard or a, a actual cap.
• 16:21
  Jen: Yeah.
• 16:21
  I think you
• 16:21
  Matt: should get, like, a cape.
• 16:23
  Chris: Yes.
• 16:25
  Not all, not all Captain Sensibles wear capes.
• 16:27
  Not
• 16:28
  Jen: capes.
• 16:28
  No, but maybe just a cap.
• 16:31
  Chris: Um, I forgot what the
• 16:33
  Jen: Captain Sensible bit was.
• 16:34
  About kidneys, maybe?
• 16:34
  Chris: Yes.
• 16:35
  On a serious note, well, thank you.
• 16:37
  You remembered.
• 16:37
  Um, if you've got a kidney disease or if you're on a restricted diet for
• 16:42
  medical reasons, protein targets are really a conversation for your doctor
• 16:46
  because they're so individual, you can do a lot of damage by just going
• 16:50
  with generic numbers thrown about- Mm
• 16:52
  the internet.
• 16:52
  So just something to keep in mind and be careful of.
• 16:55
  Jen: Thank you, Captain.
• 16:56
  Okay, let's chat about creatine, because that has had so much, and is still
• 17:01
  getting so much, of attention, isn't it?
• 17:04
  Chris: It is, and I think rightly, too.
• 17:06
  And, um, there was a 2025 systematic review and meta-analysis of kind of papers
• 17:12
  and publications, and it found that, um, creatine plus resistance training improved
• 17:18
  lower limb strength and lean muscle mass far more than just training alone.
• 17:23
  Matt: More than training alone is an important phrase, because
• 17:26
  creatine is not a, a replacement.
• 17:29
  It is an addition.
• 17:30
  Jen: Yeah.
• 17:31
  And the effects, um, in those trials, they were seeing the most, you know,
• 17:35
  the most visible effects were in the first 32 weeks, and it actually looked
• 17:40
  the same across, you know, men and women and different fitness levels as well.
• 17:44
  Matt: Mm.
• 17:44
  Chris: Yeah.
• 17:45
  And, um, although evidence in people over 90 who are still fairly is, is
• 17:49
  genuinely limited, it's a, it's a gap worth noting, but it's, this is
• 17:53
  not an unmeasured benefit, is it?
• 17:55
  So we've seen this across groups.
• 17:57
  It's fair to say there aren't that many people over 90 maybe who go to
• 17:59
  the gym who the, the studies were able to target, but I think creatine as a
• 18:04
  whole is really interesting, isn't it?
• 18:05
  And-
• 18:05
  Jen: Mm.
• 18:06
  Chris: Mm … I think for the, the elderly, there's a lot more to go
• 18:09
  in terms of studies on creatine.
• 18:10
  So
• 18:10
  Jen: watch this space.
• 18:11
  Chris: Yeah.
• 18:11
  Jen: Vegetology
• 18:12
  Matt: creatine, coming soon.
• 18:15
  Chris: Yeah, the, oh, the product development team, they're ready.
• 18:16
  The, the whiteboard over Matt's shoulder's now-
• 18:19
  Matt: Yeah … the board.
• 18:20
  Team creatine.
• 18:21
  Maybe we could do it all together
• 18:24
  Jen: That would taste…
• 18:24
  No, that wouldn't taste good.
• 18:27
  Okay.
• 18:27
  So moving on from creatine, let's talk about aerobic activity.
• 18:31
  So that burns, obviously, energy directly, and also supports your cardiovascular
• 18:38
  health, um, but is beneficial alongside strength work, um, like weightlifting.
• 18:44
  So I think the recommended is at least 150 minutes a week.
• 18:49
  Matt: Yeah, and that doesn't have to be, like, a formal exercise,
• 18:52
  like, you know, like on a treadmill.
• 18:53
  Jen: No.
• 18:54
  Chris: No, that's right.
• 18:55
  So walking, uh, cycling, swimming, they all count.
• 18:59
  Um, and so does the kind of low-level activity that we talked about earlier
• 19:02
  that you do without even thinking about, be it the, the picking up the shops, uh,
• 19:05
  the shopping- Yeah, picking up the shops
• 19:07
  sorry, the groceries.
• 19:09
  Jen: Yeah.
• 19:09
  Tell you what, if you can manage that at the age of 90, Guinness Book of Records.
• 19:13
  Chris: Yeah.
• 19:13
  No creatine required if you, if you can do that.
• 19:15
  Uh, but those things, they add up way more than people kind
• 19:18
  of maybe expect or assume.
• 19:20
  Jen: So we've covered some of the more obvious things, but then there's
• 19:22
  the easy things that people, well, usually ask about that, um, to help
• 19:27
  boo- boost metabolism, like caffeine, which is the obvious one, and green
• 19:32
  tea is another one, probably due to its antioxidant properties.
• 19:36
  Matt: Go on then, break my heart.
• 19:37
  Come on.
• 19:38
  Give me the lowdown on caffeine.
• 19:40
  Chris: Well, there is a measurable effect
• 19:44
  of caffeine.
• 19:44
  Woo!
• 19:44
  That's the good news, because studies have shown that, um, it-
• 19:48
  caffeine does transiently increase energy expenditure and fat oxidation
• 19:53
  under sedentary conditions.
• 19:55
  So if you- Wow.
• 19:56
  Right … sit around, it's doing a job.
• 19:57
  Matt: Yeah, yeah.
• 19:58
  Jen: But…
• 19:58
  Chris: There's always a but.
• 20:00
  I didn't do the but this time.
• 20:01
  Yeah.
• 20:03
  It's normally Prince of Darkness over here who does the but
• 20:07
  Jen: But the effect sizes are small and they are quite short-lived
• 20:11
  and they're nowhere near the league of like building muscle.
• 20:14
  So they're real, but they're quite modest.
• 20:17
  Chris: Yeah.
• 20:17
  So they are a nice little temporary thing, but they're not exactly a strategy, you
• 20:21
  know, being just caffeine or green tea.
• 20:23
  Matt: Mm.
• 20:24
  Chris: Do you like green tea?
• 20:25
  No.
• 20:25
  I, I've got into green tea.
• 20:26
  I've… Yeah, I drink it.
• 20:27
  Matt: No, I just, I just prefer normal tea.
• 20:29
  Jen: I feel like it's something you tolerate.
• 20:31
  Matt: Yeah.
• 20:32
  Yeah, I've just not built up the resistance for
• 20:34
  Chris: it.
• 20:34
  Actually, I have to drink decaffeinated green tea, 'cause it-
• 20:37
  Jen: Rock and roll.
• 20:38
  Matt: So, so you're definitely not gonna benefit from this study then are you?
• 20:43
  Chris: No, but luckily green tea's full of antioxidants, which are, are good.
• 20:46
  Ah, okay.
• 20:47
  They, they do the same thing, so it's the kind of healthier version.
• 20:48
  He'll be
• 20:48
  Jen: laughing when he's 90.
• 20:49
  Matt: Yeah, yeah, yeah.
• 20:51
  Metabolic rate- Yeah … still faster than the speed of sound.
• 20:53
  Chris: No joy, 'cause I'm just drinking- … bitter green tea, but there you go.
• 20:59
  Um, but here's something else that surprises people, and another one
• 21:03
  potentially for the fact fans out there, that cutting calories hard doesn't raise
• 21:08
  metabolic rate as a lot of people believe.
• 21:10
  Mm. So more, um, calorie restriction generally lowers energy expenditure.
• 21:16
  Matt: Yeah.
• 21:16
  It is, I think, a little bit opposite to what everyone hopes for, isn't it?
• 21:19
  When they cut calorie diets, they hope, uh, cut their calories, it, they
• 21:22
  hope that it's gonna increase their-
• 21:24
  Jen: Yeah,
• 21:24
  Matt: yeah … metabolic
• 21:24
  Jen: rate.
• 21:24
  Yeah, it is.
• 21:26
  Um, calorie restriction, it can improve how your mitochondria work,
• 21:30
  at least in lab studies, but that's, that's about keeping cells healthy.
• 21:35
  It doesn't necessarily mean you're burning more calories day to day.
• 21:40
  Matt: Yeah.
• 21:40
  So for the metabolic rate specifically, the heavi- heaviest evidence then sort
• 21:45
  of sits with training and protein rather than supplements or your green tea.
• 21:50
  Jen: Yeah.
• 21:50
  Chris: I think that's fair.
• 21:51
  Um, let- let's say with the potential addition of creatine, because
• 21:54
  that is a well-supported extra.
• 21:56
  Jen: Mm.
• 21:57
  Chris: Um, but it's, it's worth keeping in mind that the, the gains are incremental.
• 22:01
  Uh, they're not things that are just gonna happen overnight, so you've,
• 22:04
  you've gotta kinda bear with these pro- kind of processes or dietary choices.
• 22:08
  Jen: Yeah.
• 22:09
  Okay.
• 22:10
  So why does our metabolism slow with age?
• 22:14
  Chris: So after 60, when it genuinely does slow by just under 1% a year,
• 22:21
  muscle loss drives this slowing down, but so too does mitochondria, which
• 22:24
  is making less ATP efficiently, uh, by anabolic hormones falling away and by
• 22:31
  the tissue that's left doing less per kilogram of body weight than it used to.
• 22:36
  Matt: And before 60, it mostly doesn't, which means the thing that
• 22:40
  I've been muttering about since I was in my mid-30s, well, it's kinda
• 22:45
  got a solid alibi, hasn't it, really?
• 22:47
  Jen: Yes, a very solid one.
• 22:48
  6,400 people, 29 countries, case dismissed.
• 22:53
  Matt: All right.
• 22:53
  All right, numbers girl over there.
• 22:55
  I've been blaming a complete bystander, I suppose, for 10 years.
• 22:59
  Chris: Yep, and it's fair to say the, um, the practical version's very short.
• 23:03
  Keep the muscle you've got and feed it properly.
• 23:06
  Your body stays responsive to training and diet far longer, far
• 23:09
  longer into life than people assume.
• 23:12
  Jen: So that's it for this week.
• 23:13
  Make sure you hit that subscribe button.
• 23:16
  We're gonna keep unpacking the science behind everyday questions.
• 23:19
  Tell us in the comments what you'd like us to explore next.
• 23:23
  Thanks for geeking out with us, but for now, that's it from me.
• 23:26
  Matt: That's it from me.
• 23:27
  Chris: And that's it from me.
• 23:28
  Jen: Bye for now.